Assessment of Herbicide Transport in Central Missouri Watersheds Using a Process-Based Index Model.

Identification of areas at high risk for pesticide loss is a necessary requirement for targeting best management practices (BMPs) at the most vulnerable areas within fields or watersheds. To work toward that goal, a process-based index model has been developed to assess relative landscape vulnerability to hydrologic losses of pesticides. The model applies mathematical functions to assign scores (i.e., degree of risk) on the basis of pesticide, soil, and landscape properties relevant to the environmental fate of pesticides. The model uses the NRCS county SSURGO (Soil Survey Geographic) database as the input source of soils data. The risk of pesticide transport is considered for three hydrologic pathways, or scenarios: leaching, solution runoff, and particle adsorbed runoff. The model is used to compute the relative risk of pesticide transport for a given pesticide for each hydrologic pathway, providing the spatial and temporal risk of transport. The model is being applied to the Youngs Creek watershed in the Central Claypan Region of Missouri, a watershed with known herbicide contamination problems. The risk of herbicide loss for each hydrologic pathway is determined for each of four corn herbicides (atrazine, metolachlor, isoxaflutole, and glyphosate). By computing area-weighted scores at the watershed scale, a score for each herbicide and hydrologic pathway scenario combination is determined. The area-weighted scores are used to evaluate the potential spatial and temporal risk of applying these herbicides in the Youngs Creek watershed. The information provided by the model is then used to make recommendations regarding the choice of herbicides that will minimize the risk of hydrologic transport for this watershed.

 Keywords: index model, herbicide transport, leaching risk, runoff risk.